1. Field of the Invention
This invention relates to the removal of sulfur dioxide from industrial waste gas, for example combustion gas from steam power plants, by wet scrubbing the gas in a horizontal, elongate, gas-liquid contactor with an aqueous absorbent.
2. Description of the Prior Art
Horizontal, elongate spray scrubbers devoid of internal packing are effective gas-liquid contactors for removal of sulfur dioxide from large volume flows of waste gas. A particularly effective scrubber of this type utilizes aqueous absorbent sprays directed across the chamber substantially perpendicular to the horizontal flow of waste gas as more fully described in U.S. Pat. No. 3,948,608 to Alexander Weir, Jr. A commercial embodiment of this scrubber has a plurality of spray nozzles positioned at the top of the scrubber as illustrated in FIG. 1. The nozzles are arranged in stages as illustrated in FIG. 2 and direct aqueous absorbent substantially vertically downward across cross sections of the gas flow path along the length of the scrubber. Typically, from four to six stages are used. Individual headers convey absorbent to the nozzles of each stage in an amount necessary to satisfy the gas/liquid flow rate ratio (G/L) required for the particular installation. This amount may be, for example, 700 liters per second in each stage discharged through 50 nozzles having 90 degree cone angles spaced 0.163 meters apart along the individual header.
The sulfur dioxide removal or absorption efficiency of these horizontal scrubbers is a function of many variables as reported in our article entitled "The Kellogg-Weir Air Quality Control System", Chemical Engineering Progress, pages 64-65, (August 1977) incorporated herein by reference. In summary of relevant aspects of that article, we express the following relationships: ##EQU1## where: D=diffusivity of SO.sub.2 in the gas phase
G=gas volume flow rate PA1 K.sub.g =overall mass transfer coefficient PA1 L=liquid volume flow rate per stage PA1 N=number of spray stages PA1 P=outlet concentration of SO.sub.2 in the waste gas PA1 P.sub.o =inlet concentration of SO.sub.2 in the waste gas PA1 R=gas constant PA1 T=gas temperature PA1 d=(Sauter) mean diameter of the spray droplets PA1 k.sub.g =gas phase mass transfer coefficient PA1 k.sub.l =liquid phase mass transfer coefficient PA1 l=mean distance traveled by the spray droplets PA1 m=slope of the equilibrium curve characterizing the gas/liquid pair PA1 u=relative velocity between the spray droplets and the gas PA1 v=mean velocity of the spray droplets.
The relative importance of k.sub.g and k.sub.l varies not only according to the choice of absorbent, but also varies according to the sulfur dioxide concentration existing at any point along the waste gas flow path.
For example, the use of a very effective absorbent such as a 5 weight percent sodium carbonate solution results in little or no liquid phase mass transfer resistance and, as stated in the reference article, m=0. In this circumstance, equation (2) becomes: ##EQU2##
On the other hand, the use of a relatively ineffective absorbent such as a calcium carbonate slurry results in high liquid phase mass transfer resistance throughout most or all of the longitudinal waste gas flow path primarily because of slow dissolution of calcium carbonate in water and resulting lower absorption efficiency.
Mass transfer characteristics of other absorbents are generally between the above-mentioned extremes. Quite commonly, a particular system will be liquid phase mass transfer limited proximate the scrubber gas inlet and gas phase mass transfer limited proximate the gas outlet because of the decreasing sulfur dioxide concentration along the waste gas flow path.
Referring to equation (6), one might expect that in a given horizontal scrubber, the efficiency of sulfur dioxide removal will be proportionately increased by increasing the liquid rate in each stage and/or by increasing the number of spray stages. Contrary to expectation, we have found in gas phase mass transfer limited regions that increases in the number of spray stages and/or liquid flow rate do not bring about corresponding increases in sulfur dioxide removal.
We have now found that this anomaly is caused by mutual interference of spray droplets from proximate spray nozzles. These droplets collide and coalesce at the initial horizontal plane of interference and for some distance below that plane until a point is reached where substantially all of the droplets fall parallel with each other and no significant further interference occurs. In the course of travel, droplet mean diameter increases significantly, as much as by a factor of 4, from the initial droplet diameter prior to interference. The increased droplet size results in significant reduction in gas-liquid contact area which, in turn, results in decreased scrubbing efficiency according to equations (1) through (6).
This problem could be avoided by the use of sprays which do not interfere with each other. In view of the large spray volume rate previously recited, however, it is quickly apparent that a horizontal scrubber designed without spray interference would be impractically large.
We have additionally found that some spray interference can exist without significant detrimental effect on the resulting droplet size. To quantify this phenomenon, we express the extent of spray interference by the term "interfering spray density" (I.S.D.) calculated as the average aqueous absorbent flow rate per unit area at any horizontal plane. A method for this calculation is recited later in this specification. The interfering spray density (I.S.D.) attains a maximum value at a short distance below the horizontal plane of initial interference of the spray droplets. Most importantly, we have found that the detrimental effects of spray interference may be significantly reduced by maintaining the I.S.D. below a critical value.